We used the mRNA differential.display to extract atrial specific genes against ventricles in rats, because such genes might explain the atrial specific structure and function. One of the extracted genes is identified as the nebulin cDNA by sequencing. Nebulin, a component of a cytoskeletal matrix that coexists with actin and myosin filaments, is a protein in the sarcomere of skeletal muscles. Although nebulin is not believed to exist in cardiac muscle, we confirmed nebulin mRNA was expressed strongly in skeletal muscles, moderately in atria, but not in ventricles or other organs by dot blot. The length of the atrial nebulin mRNA is much the same as the skeletal in northern blot. To delineate the functional significance of nebulin in atria, we observed the developmental change of the mRNA expression. Atrial or ventricular RNAs were purified from day 16 and day 20 embryos, 7 days old neonates, 4 weeks old rats, and 6 , months old rats. Nebulin mRNA was quantitated by dot blot, and corrected by GAPDH.Nebulin mRNAs were recognized in all stages of atria, but not in ventricles. They were expressed lower in the embryonic, and higher (3.8 fold vs day16 embryo) in 4 w/o atria. Since nebulin is considered to connect the thin filament to Z-line in the sarcomere, the expression of actin mRNA was also observed. Actin mRNA was strongly expressed both in atria and ventricles. Actin mRNAs in atria increased its expression during embryonic period, then decreased in 6 m/o atria. In conclusion, nebulin mRNA is expressed not only in skeletal muscles, but also in atria. The expression of mRNAs were independently regulated between nebulin and actin. Since the expression of nebulin mRNA change developmentally in atria, it may play a certain role in atrium to characterize its muscle function.